Create todos for epics

Summary

We have project validation and NOT NULL constraint on Todo. That doesn't allow us to create todos for objects without project. So far I am aware of at least personal snippets (no project, no group) and epics (parent is group) will have that problem too.

Suggestion

Remove the project constraint and validate only when the object belong to a project. Create todos for mentioned users on epics

Note

Creating todos from Issuables UpdateService could be extracted to another class - we have quite a lot of code duplication there.